Answer
An unfavorable environment shows that existing genetic combinations may no longer be successful. The increased diversity generated by meiosis/recombination available to sexually reproducing organisms and would be a selective advantage to them during evolution as it creates the opportunity for more successful genetic combinations to emerge.
Work Step by Step
The key here is to see that a changing or changeable environment provides stresses which could be met by some genetic combinations generated by meiosis-- this is why sexual evolution evolved and provides an advantage today in some situations.
